The present invention relates to a digital communication apparatus.
In the communication field, a spread-spectrum communication technique is suitable for a high-speed data transmission in the environment where the channel characteristics such as multipath fading undergo a considerable dynamic change.
Typical examples of spread-spectrum communication technique include a direct spread (DS) system and a frequency hopping (FH) system. The DS system is advantageous in view of small circuit size and high-speed data transmission, while the FH system is advantageous in view of channel capacity and communication reliability. Examples of the FH system include a high-speed FH system and a low-speed FH system. The high-speed FH system in which communication is made while the carrier frequency is being switched in a short period of time, is considerably increased in hardware size as compared with the low-speed FH system, but is advantageous in view of reliability against multipath fading.
Examples of a primary modulation in the FH system include a frequency shift keying (FSK) modulation, a phase shift keying (PSK) modulation and the like. In view of simplicity in circuit configuration requiring no phase control, the FSK modulation is relatively often used.
According to an arrangement of an FH digital communication apparatus of prior art, the transmission throughput per channel, even for one-channel communication, is the same as that in communication using a plurality of channels.
According to another arrangement of the FH digital communication apparatus of prior art, carrier frequency waveforms are synthesized by a PLL synthesizer in the transmitter. This makes it difficult to switch the carrier frequency at a high speed of the order of micro second. Thus, such an arrangement is not suitable for the high-speed FH system. Further, the receiver requires, at its envelop line detector unit, analog band-pass filters having sharp amplitude characteristics in number equal to the number of carrier frequencies. This results in an increase in hardware. To achieve the high-speed FH system, it would be proposed that both the generation of waveforms and the detection of frequencies are conducted by a digital signal process. However, this disadvantageously excessively increases the frequency of a sampling clock for a digital signal process. On the other hand, when detecting frequencies using a discrete Fourier transform (DFT), it is required that the DFT operation interval is accurately in synchronism with the time slot. This has hitherto been difficult.
There is known a digital communication apparatus using a code multiplexing MFSK modulation using M carrier frequencies, M being an integer not less than 4. According to D. J. Goodman et al., xe2x80x9cFrequency-Hopped Multilevel FSK for Mobile Radioxe2x80x9d, Bell System Technical Journal, Vol. 59, No. 7, pp. 1257-1275, September 1980, M frequencies (tones) are prepared in a predetermined band according to the high-speed FH system, and a unique code is assigned to each user on a time-frequency matrix. However, a high sampling rate is required in the DFT process, making it practically difficult to achieve the hardware.
There is now considered a digital communication apparatus of the mode changeover type arranged to make a frequency multiplex communication with either the MFSK or FH mode selected according to multiplicity. However, when the transmitter is not provided with a data scrambling function and the appearance probability of transmission data is uneven, the spectra of a transmission signal are also uneven. Further, when specific frequency components appear continuously, timing extraction becomes difficult in the receiver. This lengthens the time required for pulling into synchronism. Further, in the receiver, there are instances where, in an operation mode according to the MFSK mode, a plurality of reception signals are detected under the influence of noise, a spurious response or the like. In such a case, the maximum likelihood word cannot be determined. Further, in an operation mode according to the FH mode, too, when a plurality of words are calculated by a majority judgment, the maximum likelihood word can neither be determined.
In G. Einarsson, xe2x80x9cAddress Assignment for a Time-Frequency-Coded, Spread-Spectrum Systemxe2x80x9d, Bell System Technical Journal, Vol. 59, No. 7, pp 1241-1255, September 1980, two methods are proposed for generating hopping codes from data in a digital FH-MFSK communication system. One is based on the premise of a synchronous system, while the other is based on the premise of an asynchronous system (a code multiplexing system providing a chip synchronism between users, but not providing a frame synchronism between users). Both methods are based on a Reed-Solomon code. However, under the influence of frequency-selective fading, there might occur a miss detection (deletion) of all specific frequency components.
It is an object of the present invention to provide a digital communication apparatus in which a high-speed data transmission is made using a multilevel frequency shift keying (MFSK) modulation mode when all the channels become vacant.
It is another object of the present invention to provide a digital communication apparatus in which a data communication is made according to a high-speed FH mode with no considerable increase in both sampling clock frequency and hardware size even though reception carrier frequencies are detected by a DFT operation unit in the receiver.
It is a further object of the present invention to provide a digital communication apparatus in which, using a low sampling-rate DFT processor capable of processing a xc2xd band width of a sub-band, a specific sub-band is modulated/demodulated according to the MFSK or code multiplexing MFSK mode even in the environment where simultaneous communications are made using a plurality of sub-bands.
It is still another object of the present invention to provide a digital communication apparatus of the mode changeover type capable of randomizing transmission data without use of a scrambler and having maximum likelihood word determining means.
It is a still further object of the present invention to provide a digital communication apparatus highly invulnerable to fading such that random hopping codes are acquired.
To achieve the objects above-mentioned, the present invention provides a digital communication apparatus to be used for a communication system in which a plurality of digital communication apparatus share a time slot (network synchronization) and in which, using N carrier frequencies out of M carrier frequencies per time slot, an N-channel frequency multiplex communication is made with an MFSK modulation mode selected when N is equal to 1 and with an FH modulation mode selected when N is not less than 2, each of N and M being an integer. More specifically, the digital communication apparatus of the present invention comprises: the following receiver comprising a signal processing unit, a channel detection unit and a decoding unit; and the following transmitter comprising a coding unit, a channel generation unit and a waveform generation unit. In the receiver, the signal processing unit is arranged such that, when a reception signal is entered through a transmission line, there are calculated, for the reception signal, the spectrum intensity values of the M carrier frequencies per time slot, and that the spectrum intensity values thus calculated are supplied to the channel detection unit. The channel detection unit is arranged such that, when the spectrum intensity values are entered from the signal processing unit, channels are detected based on the spectrum intensity values, that the time slot is controlled in phase, that either the MFSK or FH modulation mode is selected and that reception code data for the channels are supplied to the decoding unit. The decoding unit is arranged such that, when reception code data are entered from the channel detection unit, the reception code data are decoded according to the modulation mode selected by the channel detection unit, and that reception information data are supplied. In the transmitter, the coding unit is arranged such that, when transmission information data are entered, the transmission information data are coded according to the modulation mode selected by the channel detection unit and that transmission code data are supplied to the channel generation unit. The channel generation unit is arranged to assign channels to the transmission code data received from the coding unit, to select carrier frequencies for the channels and to supply the carrier frequencies thus selected to the waveform generation unit. The waveform generation unit is arranged to supply, as a transmission signal, the signal waveforms of the carrier frequencies selected by the channel generation unit, the transmission signal being supplied, in synchronism with the time slot, to the transmission line. According to the digital communication apparatus having the arrangement above-mentioned, the modulation mode can be switched from the FH mode to the MFSK mode and vice versa merely by changing the contents to be processed in the coding and decoding units. This enables the FH or MFSK mode to be used as properly selected according to the usage of channels. This achieves an efficient high-speed data transmission without reliability lost.
In a digital communication system using another digital communication apparatus according to the present invention, a plurality of digital communication apparatus share a time slot (network synchronization) and a frequency multiplex communication is made with carrier frequencies out of M carrier frequencies selected, per time slot, for a plurality of channels, M being an integer not less than 2. This digital communication apparatus comprises; the following transmitter comprising a frequency selection unit and a waveform generation unit; and the following receiver comprising a down-converter unit, a DFT operation unit, a threshold judgment unit, a synchronizing signal generation unit, a latch unit and a decoder. In the transmitter, the frequency selection unit is arranged to determine, for entered transmission data, carrier frequencies to be used out of the M carrier frequencies per log2 M bits according to a conversion table. The waveform generation unit is arranged to supply, in synchronism with the time slot, frequency waveforms corresponding to the carrier frequencies to be used, the frequency waveforms being supplied, as a transmission signal, to the transmission line for each period of one time slot T. In the receiver, the down-converter unit is arranged such that a reception signal entered through the transmission line is down-converted in frequency to a low frequency band. The DFT operation unit is arranged to successively execute, per sampling clock period xcex94t, a discrete Fourier transform (DFT) for a period of the latest one time slot (T=Nxc3x97xcex94t) on the signal after down-converted in frequency, thereby to respectively calculate spectrum values I (k) (k=1, 2, . . . , M) for the M carrier frequencies, N being an integer not less than M. The threshold judgment unit is arranged to detect, out of the M carrier frequencies, carrier frequencies of which spectrum values I(k) exceed a threshold value, these carrier frequencies being detected as candidate carrier frequencies per sampling clock period xcex94t. The synchronizing signal generation unit is arranged to generate, based on the spectrum values I(k) and the candidate carrier frequencies, a synchronizing trigger signal for synchronization with the time slot. The latch unit is arranged to determine, as reception carrier frequencies, the candidate carrier frequencies at the time of assertion of the synchronizing trigger signal. The decoder is arranged to supply, based on a conversion table identical with that in the frequency selection unit, log2M-bit reception data for the reception carrier frequencies. According to the digital communication apparatus having the arrangement above-mentioned, a transmission signal can be pulled, using the results of a DFT operation, into accurate synchronism with the time slot. Thus, such a highly precise frequency detection suitable for the high-speed FH system achieves a highly reliable data communication with a high frequency-utilization efficiency.
The present invention provides a further digital communication apparatus using either an MFSK modulation mode or a code multiplexing MFSK modulation mode, using M carrier frequencies per sub-band, M being an integer not less than 4, and this digital communication apparatus is arranged such that the M carrier frequencies per sub-band are orthogonally disposed at frequency intervals not less than 2/T in which T is a frequency switching period of time. According to the digital communication apparatus having the arrangement above-mentioned, using a low sampling-rate discrete Fourier transform capable of processing a xc2xd band width of a sub-band, frequencies in the sub-band around a specific frequency can be detected even in the environment where simultaneous communications are made using a plurality of sub-bands.
The present invention provides a further digital communication apparatus using either an MFSK modulation mode or a code multiplexing MFSK modulation mode, using M consecutive carrier frequencies randomly selected per predetermined time interval L, M being an integer not less than 4, and this digital communication apparatus is arranged such that the time interval L is a value equal to the product of a frequency switching period of time T and a positive integer and that the M carrier frequencies are orthogonally disposed at frequency intervals not less than 2/T. According to the digital communication apparatus having the arrangement above-mentioned, using a low sampling-rate discrete Fourier transform capable of processing a xc2xd band width of a sub-band, frequencies in the sub-band around the desired frequency can be detected even in the environment where simultaneous communications are made using a plurality of sub-bands.
The present invention provides a further digital communication apparatus using either an MFSK modulation mode or a code multiplexing MFSK modulation mode, using M carrier frequencies per sub-band, M being an integer not less than 4, and this digital communication apparatus comprises a transmitter and a receiver. The receiver comprises: N diversity branches in which signals received from N points spatially separated from the diversity branches, are respectively down-converted in frequency to low frequency bands, thereby to supply N-sequence base band signals, N being an integer not less than 2; a frequency detection unit formed of M operation units for respectively calculating the signal levels of the M carrier frequencies; a selector for assigning the N-sequence base band signals to the M operation units; and a timer for controlling the selector to change the base band signal to be assigned to a specific operation unit out of the M operation units when the signal level calculated by the specific operation unit does not exceed a threshold level in a predetermined period of time. According to the digital communication apparatus having the arrangement above-mentioned, signal reception can be made with no fading influence in each of the operation units.
The present invention provides a further digital communication apparatus to be used for a digital communication system in which a plurality of digital communication apparatus share a time slot and in which a half-duplex data communication is made using either an MFSK modulation mode or a code multiplexing MFSK modulation mode, and this digital communication apparatus comprises a transmitter and a receiver which share a single antenna. In this digital communication apparatus, the receiver comprises: first means for storing, as a reference phase error, a phase error which is present immediately before the communication mode is switched from the reception mode to the transmission mode; and second means for generating, after the reception mode has been switched to the transmission mode, a regenerative synchronizing signal for synchronous control of the time slot, using a feedforward control based on the stored reference phase error, and for supplying the regenerative synchronizing signal thus generated to the transmitter. According to the digital communication apparatus having the arrangement above-mentioned, it is possible to maintain a network synchronization at the time when there is made, using the common antenna, a code division multiple access (CDMA) as done in an FH-MFSK mode in the same frequency band.
The present invention provides a further digital communication apparatus comprising: a transmitter in which a convolutional coder and an interleaver are combined to code transmission data without use of a scrambler; and a receiver in which a majority decoder is used to execute a most likelihood word decoding. In this digital communication apparatus, using M carrier frequencies per time slot, a frequency multiplex communication is made with either an MFSK modulation mode or an FH modulation mode selected according to multiplicity, M being an integer not less than 2. This digital communication apparatus comprises (i) the transmitter comprising: the convolutional coder for supplying a convolutional code sequence according to an input information sequence; the interleaver for supplying an interleave sequence according to the convolutional code sequence; an FH coder for supplying an FH code sequence according to the interleave sequence; a first switching unit for supplying, according to a switching signal, either the interleave sequence or the FH code sequence as a transmission sequence; and an M-ary independent signal transmitter unit for supplying, per time slot, a transmission signal containing, out of M mutually independent frequency components, one frequency component corresponding to the transmission sequence, and (ii) the receiver comprising: an M-ary independent signal receiver unit for supplying a threshold judgment pattern generated by making a threshold judgment on each of the intensity values of M frequency components of a reception signal; an operational mode control circuit for judging the multiplicity based on the threshold judgment pattern and for supplying the switching sinal according to the multiplicity; an FH decoder for supplying an FH decoding pattern according to the threshold judgment pattern; a second switching unit for selecting, according to the switching signal, either the threshold judgment pattern or the FH decoding pattern; the majority decoder for supplying a majority decoding sequence according to the pattern selected by the second switching unit; a deinterleaver for supplying a deinterleave sequence according to the majority decoding sequence; and a Viterbi decoder for supplying an information sequence according to the deinterleave sequence. According to the digital communication apparatus having the arrangement above-mentioned, both the convolutional coder and the interleaver encode transmission data, causing the transmission data to be randomized without use of a scrambler. This not only equalizes the spectra of a transmission signal, but also reduces the frequency in continuous appearance of specific frequency components. Further, the majority decoder in the receiver makes a majority judgment on each of the bits forming a word, thus determining the most likelihood word.
The present invention provides a further digital communication apparatus comprising a frequency hopping generator (FH coder) comprising the following conversion means and the following operation means. More specifically, the conversion means is arranged to convert a data value x which is an element of a Galois field, into a code w which is a non-zero element of the Galois field, according to the following conversion equation using a function f:
w=f(x)
when the number M of values which a data can present, is equal to 2k (k is a positive integer) and the number Q of the elements of the Galois field is equal to pr ( greater than M) in which p is a prime number and r is a positive integer. The operation means is to arrange to calculate, according to the code w, a hopping code vector {circumflex over ( )}y composed of L components using the following Galois operation:
{circumflex over ( )}y=w xc2x7{circumflex over ( )}xcex1+i xc2x7{circumflex over ( )}e
wherein i is the user identification No. which is an element of the Galois field; xcex1 is one of the primitive elements of the Galois field; {circumflex over ( )}xcex1 is a spread code vector of L components and is equal to (1, xcex1, xcex12, . . . xcex1Lxe2x88x921) in which L is an integer not less than 2 and not greater than prxe2x88x921; and {circumflex over ( )}e is a unit vector of L components and is equal to (1, 1, . . . , 1). According to the digital communication apparatus having the arrangement above-mentioned, Q is greater than M and the data value x is previously converted into the non-zero code w, based on which the hopping code vector {circumflex over ( )}y is calculated.